In the following, three key points will be explained, which constitute central technical problems in the development and successful economic exploitation.
A solar cell converts the energy of light into electrical energy. In contrast to inorganic solar cells, free charge carriers are not created directly by the light in the case of organic solar cells, but instead bound Frenkel excitons first form, which are electrically neutral excitation states in the form of bound electron-hole pairs. These excitons can only be separated by very powerful electric fields or at suitable interfaces. Sufficiently powerful fields are not available in organic solar cells, so that all the promising concepts for organic solar cells are based on the separation of excitons at photoactive interfaces (Organic Donor-Acceptor Interface—C. W. Tang, Applied Physics Letters, 48 (2), 183-185 (1986)) or Inorganic Semiconductor Interface (cf. B. O'Regan et al., Nature 353, 737 (1991)). For this, it is necessary for excitons generated in the bulk of the organic material to be able to diffuse to this photoactive interface.
The low-recombination diffusion of excitons to the active interface therefore plays a critical role in the case of organic solar cells. In order to make a contribution to photo-electric current, the exciton diffusion length in a good organic solar cell must therefore be at least in the same range as the typical penetration depth of light so that the greater part of the light can be exploited. Organic crystals or thin films which are perfect in terms of their structure and chemical purity certainly satisfy this criterion. For large-scale applications, however, it is not possible to use monocrystalline organic materials, and the production of multiple layers with sufficient structural perfection is still very difficult, even today.
Instead of increasing the exciton diffusion length, it is also possible to reduce the mean distance from the closest interface. Document WO 00/33396 proposes the creation of an interpenetrating network: a layer contains a colloidally dissolved substance, which is distributed in such a way that a network forms via which the charge carriers can flow (percolation mechanism). In a network of this kind, the task of light absorption is performed either by only one of the components or by both.
The advantage of a mixed layer of this kind is that the excitons produced only have to travel a very short distance before they reach a domain boundary, where they are separated. The electrons and holes are transported away separately in the dissolved substance or in the rest of the layer. Since the materials in the mixed layer are in contact with one another everywhere, it is decisive with this concept that the separated charges should have a long life on the material concerned and that closed percolation paths are available from every location for both charge carrier locations to the respective contact. With this approach, it was possible to achieve efficiencies of 2.5% for polymer-based solar cells produced by wet-chemical means (C. J. Brabec et al., Advanced Functional Material 11, 15 (2001)), while polymer-based tandem cells already have an efficiency of more than 6% (J. Y. Kim et al., Science 13, 222-225 (2007)). Other known approaches for achieving or improving the properties of organic solar cells are listed below: